Just to say thank you, I appreciate all of your comments.
As some have identified, the object is to not be an alarmist over an issue that may not be that critical, but alerting the individual of the implications (and they do exist) is equally important. As I indicated, everything after that point is their decision. Those express my main (conflicting) thoughts.
The individual in question seemed pleased with their US roots, even though they had no intention of pursuing them. Unfortunately, the children’s desires to explore those roots could inadvertently create some unwelcome surprises. It appeared to be more of a curiosity than an intention. If they realised the possible consequences, they might be less enthusiastic.
I feel slightly ‘peeved’ to be placed in this situation, no matter how trivial, thanks to an imprudent US Congress. It also crossed my mind that FATCA could hold some unpleasant consequences for the accidental American. (camoscato: in current form, it’s similar to Finland demanding that every non-Finnish military in the world identify, and notify Finland of, all male residents in their jurisdiction under the age of 25 with any Finnish connections, who are therefore assumed to have been, or may be, evading military conscription in Finland. It’s up to those males to prove they are not, or were not, evaders.) It’s just something to bear in mind if you stumble across the ‘blissfully unaware’ accidental American at a wedding, or a funeral.